Method and apparatus for optical wireless architecture

ABSTRACT

Embodiments of the present invention pertain to optical wireless architecture. More particularly, certain embodiments of the invention pertain to a novel method and apparatus to generate millimeter-wave signals with simple and/or low cost architecture. Simple millimeter-wave generation and dispersion-tolerant transmission is based on photonic mixing of two free-running lightwaves and self-mixing down-conversion. More particularly, heterodyne mixing of two free run lightwaves is achieved, wherein one lightwave is modulated by an external modulator driven by electrical data as one of the side-bands of a millimeter-wave signal. Optical to electrical conversion is performed and the millimeter-wave signal is broadcasted by a high-frequency antenna to a receiving side having a local oscillator with self-mixing architecture to down-convert the radio frequency to its baseband form.

FIELD OF THE INVENTION

This invention generally pertains to optical wireless architecture and in particular to a novel method and apparatus to generate millimeter-wave signals with simple and low cost architecture.

BACKGROUND

Millimeter-wave technology is a wireless technology that can provide up to multi-Gigabits per second (Gbps) wireless connectivity between electronic devices. The frequency range of millimeter-wave application is significantly higher than those used for FM radio. Millimeter-wave technology can exploit unregulated bandwidth that is available world-wide and with better efficiency and security than traditional wireless LAN frequencies. In addition, higher frequencies mean shorter wavelengths. As a result, the antenna systems can be millimeter size. However, the electrical distribution of such millimeter-wave-band radio frequency signals over air is limited due to the high transmission loss.

Millimeter-wave radio-over-fiber (RoF) techniques and systems are drawing more and more research and commercial interest in part due to the seamless integration of huge transmission bandwidth with optical fiber communication and flexible wireless access provided by optical fiber and high-frequency RF carriers: (W. Jian et al., “Energy-Efficient Multi-Access Technologies for Very-High-Throughput Avionic Millimeter Wave, Wireless Sensor Communication Networks,” IEEE J. of Lightw. Technol., Vol. 28, No. 16, pp. 2398-2405, Aug. 15, 2010); (G. Chang et al., “Broadband Access Technologies for Very High Throughput Wireless Sensor Communication Networks,” IEEE Radio and Wireless Symposium'09, Jan. 2009).

Photonic generation and up-conversion of millimeter-wave signals play important roles in millimeter-wave system design: (A. Wiberg et al., “Fiber-optic 40 GHz Millimeter-wave link with 2.5 Gb/s data transmission,” IEEE Photon. Technol. Lett., vol. 17, no. 9, pp. 1938-1940, Sep. 2005); (H. Song et al., “Error-free simultaneous all-optical upconversion if WDM radio-over-fiber signals,” IEEE Photon. Technol. Lett., vol. 17, no. 8, pp. 1731-1733, Aug. 2005).

The traditional schemes of millimeter-wave signal generation, such as double-sideband (DSB) and optical carrier suppression (OCS), experience a performance-fading problem and a limitation of transmission distance. As a result, the typical DSB millimeter-wave signal suffers severe deterioration after 40-km single-mode-fiber (SMF) transmission.

In (J. Yu et al., “Optical Millimeter-Wave Generation or Up-Conversion Using External Modulators,” IEEE Photon. Technol. Lett., vol. 18, no. 1, pp. 265-267, Jan. 2006), the generated or up-converted optical millimeter wave using OCS modulation scheme shows high receiving sensitivity, high spectra efficiency, and small power penalty. However, uneven amplitudes of optical carrier at 40 GHz after 40-km SMF transmission caused by fiber dispersion are also experienced. After 60-km SMF transmission, due to fiber dispersion and large carrier-to-sideband ratio (CSR), the eye diagram is almost closed. Furthermore, more components are adopted for millimeter-wave generation by using DSB or OCS. For example, an extra local oscillator is required for up-conversion.

Millimeter-wave signals are generally generated with a single-sideband (SSB) scheme. However, the typical millimeter-wave generation with SSB is complex. For example, an extra optical modulator or interleaver is employed in a SSB scheme. In (J. Yu et al., “A Novel Scheme to Generate Single-Sideband Millimeter-Wave Signals by Using Low-Frequency Local Oscillator Signal,” IEEE Photonics Technology Letters, vol. 20, no. 7, pp. 478-480, Apr. 1, 2008), for example, millimeter-wave is generated by using SSB modulation with a low-frequency local oscillator. Moreover, an additional external optical filter or interleaver is deployed to filter out the first-order modes, which makes the generation scheme more costly.

SUMMARY OF THE INVENTION

Briefly, the present invention describes a method of millimeter-wave signal generation that includes driving a continuous lightwave by a data signal and a modulator resulting in a data sideband signal and then combining the data sideband signal and another continuous lightwave by an optical coupler to generate a millimeter-wave signal having two non-phase-locked sidebands. The data signal is an electrical data signal optionally implementing a non-return-to-zero binary pulse modulation format or a quadrature phase shift keying modulation format, among others. Preferably, the first continuous lightwave has a wavelength of λ₁, the second continuous lightwave has a wavelength of λ₂, and the photodiode has a bandwidth larger than f_(lo), where f_(lo)=C(1/λ₁−1/λ₂) and C=3.8×10⁸. The method further includes detecting, by a photodiode achieving optical to electrical conversion, the generated millimeter-wave signal and then transmitting a corresponding high-frequency radio signal.

The present invention also describes a method of millimeter-wave signal reception that includes receiving a high-frequency radio signal that corresponds to a millimeter-wave signal having two non-phase-locked sidebands and then down-converting by a local oscillator the radio frequency signal to its baseband form. Optionally, the local oscillator has a self-mixing function.

Embodiments of the present invention also describe a millimeter-wave signal generator that includes means for driving a continuous lightwave that result in a sideband data signal and means for optically combining the sideband data signal and another continuous lightwave that result in a millimeter-wave signal having two non-phase-locked sidebands. The data signal is an electrical data signal optionally implementing a non-return-to-zero binary pulse modulation format or a quadrature phase shift keying modulation format, among others. Preferably, the first continuous lightwave has a wavelength of λ₁, the second continuous lightwave has a wavelength of λ₂, and the means for detecting has a bandwidth larger than f_(lo), where f_(lo)=C(1/λ₁−1/λ₂) and C=3.8×10⁸. The method further includes means for detecting the generated millimeter-wave signal that achieves optical to electrical conversion and means for transmitting a radio frequency signal that corresponds to the generated millimeter-wave signal.

The present invention also describes a millimeter-wave signal receiver that includes means for receiving a radio frequency signal corresponding to a millimeter-wave signal that has two non-phase-locked sidebands and means for down-converting the radio frequency signal to its baseband form. Optionally, the means for down-converting has a self-mixing function.

Embodiments of the present invention also describe a millimeter-wave signal generator that includes a modulator configured to drive a continuous lightwave by a data signal and then output a data sideband signal, and an optical coupler configured to combine the data sideband signal and another continuous lightwave and then output a millimeter-wave signal having two non-phase-locked sidebands. The data signal is an electrical data signal optionally implementing a non-return-to-zero binary pulse modulation format or a quadrature phase shift keying modulation format, among others. Preferably, the first continuous lightwave has a wavelength of λ₁, the second continuous lightwave has a wavelength of λ₂, and the photodiode has a bandwidth larger than f_(lo), where f_(lo)=C(1/λ₁−1/λ₂) and C=3.8×10⁸. The signal generator further includes a photodiode configured to detect the millimeter-wave signal and achieve optical to electrical conversion and a high-frequency antenna for transmitting a corresponding radio signal.

The present invention also describes a millimeter-wave signal receiver that includes a high-frequency antenna for receiving a radio signal corresponding to a millimeter-wave signal having two non-phase-locked sidebands and a local oscillator configured to down-convert the radio signal to its baseband form. Optionally, the local oscillator has a self-mixing function.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be realized by reference to the accompanying drawings:

FIG. 1 schematically illustrates a system and process for free-run millimeter-wave signal generation.

FIG. 2 schematically illustrates a system and process for an evaluation platform of free-run millimeter-wave signal transmission.

FIG. 3 schematically illustrates a system and process for an experimental setup of free-run millimeter-wave signal transmission.

FIG. 4 shows two configurations for clock extraction.

FIG. 5 shows optical spectra and an eye diagram of millimeter-wave generation with OCS.

FIG. 6 shows eye diagrams and bit error rate curves.

FIG. 7 is a chart depicting a process for millimeter-wave signal generation.

Embodiments of the present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which some examples of embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example to satisfy applicable legal requirements. Like numbers refer to like elements throughout.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows millimeter-wave signal generation according to embodiments of the present invention. Heterodyne mixing of two free run lightwaves is utilized. For example, two continuous lightwaves (CW₁ and CW₂) with a narrow linewidth of 100 kHz are adopted to generate the two side-bands of millimeter-wave signal, rather than the traditional one that uses optical carrier suppression (OCS) by additional modulator (MOD) or optical interleaver.

CW₁ 101, with a wavelength of λ₁, is driven by electrical data 102 with MOD as one of the side-bands of the millimeter-wave signal. CW₂ 103, with a wavelength of λ₂, is a pure continuous lightwave without any modulation as the other side-band of the millimeter-wave signal.

CW₁ and CW₂ are combined together by an optical coupler (OC) 104 to achieve millimeter-wave signal generation. As such, it is more flexible and feasible to generate any millimeter-wave in large frequency ranges. The two side-bands of the millimeter-wave signal are completely non-phase-locked with a random phase difference between them. In certain embodiments of the present invention, the relative frequency drifting between the two lightwaves will change the frequency of the millimeter-wave. Therefore, self-mixing is used to realize millimeter-wave down conversion with stable operation.

At the wireless transmitting side, the millimeter-wave signal is detected by a photodiode (PIN) 105 to achieve optical to electrical (O/E) conversion. After PIN, the millimeter-wave is transmitted through an air-link by a high-frequency antenna. At the receiving side, a local oscillator (LO) with self-mixing architecture 106 is used to down-convert the radio frequency (RF) to its baseband form. In certain embodiments of the present invention, by self-mixing, for example, the frequency drifting between the RF signals and LO can be substantially eliminated.

FIG. 2 shows an evaluation platform of free-run millimeter-wave signal generation as disclosed herein. The bit rate of data from the pseudorandom bit sequence (PRBS) pattern generator is 5 Gb/s. The optical spectrum is shown as insert (i) and the frequency of RF signal could be calculated as f_(lo)=C(1/λ₁−1/λ₂), where C=3.8×10⁸. In the FIG. 2 demonstration, f_(lo)=62.5 GHz. Because the signal is with a binary non-return-to-zero (NRZ) signal, the right side-band which carries the PRBS data after is broadened is illustrated in insert (i) other than left one. Insert (ii) shows the eye diagram of received PRBS data after 80-km SMF transmission.

Similarly, FIG. 3 shows an experimental setup of an embodiment of the present invention. A 5-Gb/s binary signal carried on 62.5 GHz optical millimeter wave was transmitted over 80-km SMF-28. A wavelength-stable continuous-wave lightwave (CW_(S)) with a narrow linewidth of 100 kHz is generated by a tunable laser at 1557.3 nm, for example, and then modulated by a Mach-Zehnder modulator (MZM) driven by a 5-Gb/s NRZ binary signal. The 5-Gb/s NRZ signal is generated by PRBS pattern generator with a length of 2¹¹.

Another pure continuous-wave lightwave (CW₂) is also generated by a wavelength-stable laser at 1557.8 nm with a narrow linewidth of 100 kHz. No phase lock between CW₁ and CW₂ is utilized. These lightwaves have equivalent optical power and random phase noise between them. The frequency stability of the laser is within 100 MHz.

After OC, the millimeter-wave RoF signal with two side-bands consists of CW₁ (the right side-band) and CW₂ (the left side-band). The optical spectrum of the millimeter-wave RoF signal is shown in FIG. 3 as insert (ii). After 80-km SMF transmission, the millimeter-wave RoF signal is detected by a PIN with a bandwidth of 70 GHz and then achieves 0/E conversion. Because the 5 Gb/s based signal is carried on the right side-band, the right side-band is wider as illustrated in insert (i) other than left one.

As before, the frequency of RF signal could be calculated as f_(lo)=C(1/λ₁−1/λ₂), where C=3.8×10⁸. In the demonstration shown in FIG. 3, f_(lo)=62 GHz. The millimeter-wave RF signal is amplified by an electrical amplifier (EA) with a bandwidth of 10 GHz. A LO with self-mixing function is deployed for down-conversion at the receiver.

Clock extraction is realized in an electrical mixer. The extraction clock is used as the LO. This LO is used to downconvert the millimeter-wave signal. The LO extraction has two configurations as shown FIG. 4. In Scheme A, the LO port in the electrical mixer has no terminator. The RF is reflected from this port, and clock is extracted in the electrical mixer. In Scheme B, the millimeter-wave is divided into two parts. The first part is connected to the RF port in the electrical mixer, and the second part is connected to the LO port in the electrical mixer. The RF cable to the LO and RF ports should be matched to get the optimal output intermediate frequency (IF) signal.

Finally, the down-converted PRBS signal is sampled and recorded by a high-bandwidth (16.5 GHz) oscilloscope operating at a sample rate of 40 GS/s. The eye diagram of received PRBS signal is shown in FIG. 3 as insert (ii). As is shown, the millimeter-wave error-free transmission over 80-km SMF is realized by using free-running generation scheme.

The significant improvement of free-running scheme is illustrated by a comparison to conventional generation schemes with DSB and OCS. Typical millimeter-wave signals generated by using DSB or OCS shows an impassable limitation at fiber transmission distance because of fiber chromatic dispersion, for example.

The optical spectra of back-to-back (B-t-B) and 80-km transmission with OCS are shown in FIGS. 5( a) and 5(b), respectively. The carrier is suppressed more than 30 dB by using an optical interleaves. The data format carried by two-sidebands is also NRZ binary pulse for a precise comparison between free-running scheme and others. FIG. 5( c) shows the eye diagram of RoF signal B-t-B transmission. After 80-km SMF transmission, the eye diagram shown in FIG. 5( d) is almost closed because of the walk-off effect of two-sidebands induced by fiber dispersion.

The bit error rate (BER) performances of millimeter-wave over RoF system by using free-running generation is measured and shown in FIG. 6. The error-free (BER=2×10⁻¹⁰) optical transmitting powers of B-t-B transmission and 80-km SMF transmission are +4.2-dBm and +4.5-dBm, which indicates the optical power penalty is only around 0.3 dB. The eye diagrams of B-t-B and 80-km SMF transmission are shown in FIG. 6 as inserts (i) and (ii) at an optical transmitting power of +4.5-dBm.

Thus, according to certain embodiments of the present invention, millimeter-wave generation utilizes optical heterodyne mixing two free-running continuous-waves and self-mixing down-conversion without any frequency locker by successfully demonstrating particular error-free transmission of 5-Gb/s PRBS data over 80-km SMF in RoF system. The chromatic dispersion tolerance of millimeter-wave signal over RoF system is significantly promoted by proposed simple and low-cost free-running generation scheme. The optical receiver sensitivities at BER=2 10-10 of particular millimeter-wave signal are +4.2-dBm (B-t-B) and +4.5-dBm (80-km SMF).

FIG. 7 generally provides a chart describing millimeter-wave signal generation according to certain embodiments of the present invention.

The foregoing descriptions illustrate and describe certain embodiments of the present invention. It is to be understood that the invention is capable of use in various other combinations, modifications, and environments; and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings and/or skill or knowledge in the relevant art.

The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Further, it should be understood that the methods and systems of the present invention are executed solely employing machines and apparatus including simple and complex computers.

Adaptations of known systems and methods that are apparent to those skilled in the art based on the description of the invention contained herein are within the scope of the claims. Moreover, later-invented or -developed equipment that carries out the methods and/or combination elements set forth in the claims are within the scope of the invention. Accordingly, the description is not intended to limit the invention to the form or application disclosed herein. All the publications cited herein are incorporated by reference in their entirety in the present application. 

1. A method of millimeter-wave signal generation, comprising: driving a first continuous lightwave by a data signal and a modulator resulting in a data sideband signal; and combining the resulting data sideband signal and a second continuous lightwave by an optical coupler, resulting in a millimeter-wave signal, said millimeter-wave signal having a first sideband and a second sideband.
 2. The method of claim 1, wherein the first sideband and the second sideband are non-phase-locked.
 3. The method of claim 1, wherein the data signal is an electrical data signal implementing one of a non-return-to-zero binary pulse modulation format and a quadrature phase shift keying modulation format.
 4. The method of claim 1, wherein the first continuous lightwave and the second continuous lightwave both have a linewidth of approximately 100 kHz.
 5. The method of claim 1, further comprising: detecting, by a photodiode, the millimeter-wave signal, wherein the photodiode achieves optical to electrical conversion; and transmitting, by a high-frequency antenna, a radio frequency signal corresponding to the millimeter-wave signal.
 6. The method of claim 5, wherein the first continuous lightwave has a wavelength of λ₁, the second continuous lightwave has a wavelength of λ₂, and the photodiode has a bandwidth larger than f_(lo), where f_(lo)=C(1/λ₁−1/λ₂) and C=3.8×10⁸.
 7. A method of millimeter-wave signal reception, comprising: receiving, by a high-frequency antenna, a radio frequency signal corresponding to a millimeter-wave signal, said millimeter-wave signal having a first sideband and a second sideband; and down-converting the radio frequency signal by a local oscillator to its baseband form.
 8. The method of claim 7, wherein the first sideband and the second sideband are non-phase-locked.
 9. The method of claim 7, wherein the local oscillator has a self-mixing function.
 10. A method of millimeter-wave signal generation and reception, comprising: driving a first continuous lightwave by a data signal and a modulator resulting in a data sideband signal; combining the data sideband signal and a second continuous lightwave by an optical coupler resulting in a millimeter-wave signal, said millimeter-wave signal having a first sideband and a second sideband; detecting the millimeter-wave signal by a photodiode, wherein the photodiode achieves optical to electrical conversion; transmitting a radio frequency signal corresponding to the millimeter-wave signal by a first high-frequency antenna; receiving the radio frequency signal corresponding to the millimeter-wave signal by a second high-frequency antenna; and down-converting the radio frequency signal to its baseband form by a local oscillator.
 11. The method of claim 10, wherein the first sideband and the second sideband are non-phase-locked.
 12. The method of claim 10, wherein the data signal is an electrical data signal implementing one of a non-return-to-zero binary pulse modulation format and a quadrature phase shift keying modulation format.
 13. The method of claim 10, wherein the first continuous lightwave has a wavelength of λ₁, the second continuous lightwave has a wavelength of λ₂, and the photodiode has a bandwidth larger than f_(lo), where f_(lo)=C(1/λ₁−1/λ₂) and C=3.8×10⁸.
 14. The method of claim 10, wherein the local oscillator has a self-mixing function.
 15. A millimeter-wave signal generator comprising: means for driving a first continuous lightwave resulting in a sideband data signal; and means for optically combining the sideband data signal and a second continuous lightwave, resulting in a millimeter-wave signal, said millimeter-wave signal having a first sideband and a second sideband.
 16. The millimeter-wave signal generator of claim 15, wherein the first sideband and the second sideband are non-phase-locked.
 17. The millimeter-wave signal generator of claim 15, wherein the data signal is an electrical data signal implementing one of a non-return-to-zero binary pulse modulation format and a quadrature phase shift keying modulation format.
 18. The millimeter-wave signal generator of claim 15, wherein the first continuous lightwave and the second continuous lightwave both have a linewidth of approximately 100 kHz.
 19. The millimeter-wave signal generator of claim 15, further comprising: means for detecting the millimeter-wave signal, wherein the means for detecting achieves optical to electrical conversion; and means for transmitting a radio frequency signal corresponding to the millimeter-wave signal.
 20. The millimeter-wave signal generator of claim 19, wherein the first continuous lightwave has a wavelength of λ₁, the second continuous lightwave has a wavelength of λ₂, and the means for detecting has a bandwidth larger than f_(lo), where f_(lo)=C(1/λ₁−1/λ₂) and C=3.8×10⁸.
 21. A millimeter-wave signal receiver comprising: means for receiving a radio frequency signal corresponding to a millimeter-wave signal, said millimeter-wave signal having a first sideband and a second sideband; and means for down-converting the radio frequency signal to its baseband form.
 22. The millimeter-wave signal receiver of claim 21, wherein the first sideband and the second sideband are non-phase-locked.
 23. The millimeter-wave signal receiver of claim 21, wherein the means for down-converting has a self-mixing function.
 24. A millimeter-wave signal system comprising: means for driving a first continuous lightwave resulting in a sideband data signal; means for combining the sideband data signal and a second continuous lightwave, resulting in a millimeter-wave signal, said millimeter-wave signal having a first sideband and a second sideband; means for detecting the millimeter-wave signal, wherein the means for detecting achieves optical to electrical conversion; means for transmitting a radio frequency signal corresponding to the millimeter-wave signal; means for receiving the radio frequency signal corresponding to the millimeter-wave signal; and means for down-converting the radio frequency signal to its baseband form.
 25. The millimeter-wave signal system of claim 24, wherein the first sideband and the second sideband are non-phase-locked.
 26. The millimeter-wave signal system of claim 24, wherein the data signal is an electrical data signal implementing one of a non-return-to-zero binary pulse modulation format and a quadrature phase shift keying modulation format.
 27. The millimeter-wave signal system of claim 24, wherein the first continuous lightwave has a wavelength of λ₁, the second continuous lightwave has a wavelength of λ₂, and the means for detecting has a bandwidth larger than f_(lo), where f_(lo)=C(1/λ₁−1/λ₂) and C=3.8×10⁸.
 28. The millimeter-wave signal system of claim 24, wherein the means for down-converting has a self-mixing function.
 29. A millimeter-wave signal generator comprising: a modulator configured to drive a first continuous lightwave by a data signal and output a data sideband signal; and an optical coupler configured to combine the data sideband signal and a second continuous lightwave and output a millimeter-wave signal having a first sideband and a second sideband.
 30. The millimeter-wave signal generator of claim 29, wherein the first sideband and the second sideband are non-phase-locked.
 31. The millimeter-wave signal generator of claim 29, wherein the data signal is an electrical data signal implementing one of a non-return-to-zero binary pulse modulation format and a quadrature phase shift keying modulation format.
 32. The millimeter-wave signal generator of claim 29, wherein the first continuous lightwave and the second continuous lightwave both have a linewidth of approximately 100 kHz.
 33. The millimeter-wave signal generator of claim 29, further comprising: a photodiode configured to detect the millimeter-wave signal and achieve optical to electrical conversion; and a high-frequency antenna for transmitting a radio frequency signal corresponding to the millimeter-wave signal.
 34. The millimeter-wave signal generator of claim 33, wherein the first continuous lightwave has a wavelength of λ₁, the second continuous lightwave has a wavelength of λ₂, and the photodiode has a bandwidth larger than f_(lo), where f_(lo)=C(1/λ₁−1/λ₂) and C=3.8×10⁸.
 35. A millimeter-wave signal receiver comprising: a high-frequency antenna for receiving a radio frequency signal corresponding to a millimeter-wave signal having a first sideband and a second sideband; and a local oscillator configured to down-convert the radio frequency signal to its baseband form.
 36. The millimeter-wave signal receiver of claim 35, wherein the first sideband and the second sideband are non-phase-locked.
 37. The millimeter-wave signal receiver of claim 35, wherein the local oscillator has a self-mixing function.
 38. A millimeter-wave signal system comprising: a modulator configured to drive a first continuous lightwave by a data signal and output a data sideband signal; an optical coupler configured to combine the data sideband signal and a second continuous lightwave and output a millimeter-wave signal having a first sideband and a second sideband. a photodiode configured to detect the millimeter-wave signal and achieve optical to electrical conversion; a first high-frequency antenna for transmitting a radio frequency signal corresponding to the millimeter-wave signal. a second high-frequency antenna for receiving the radio frequency signal; and a local oscillator configured to down-convert the radio frequency signal to its baseband form.
 39. The millimeter-wave signal system of claim 38, wherein the first sideband and the second sideband are non-phase-locked.
 40. The millimeter-wave signal system of claim 38, wherein the data signal is an electrical data signal implementing one of a non-return-to-zero binary pulse modulation format and a quadrature phase shift keying modulation format.
 41. The millimeter-wave signal system of claim 38, wherein the first continuous lightwave has a wavelength of λ₁, the second continuous lightwave has a wavelength of λ₂, and the photodiode has a bandwidth larger than f_(lo), where f_(lo)=C(1/λ₁−1/λ₂) and C=3.8×10⁸.
 42. The millimeter-wave signal system of claim 38, wherein the local oscillator has a self-mixing function. 